The entire mechanism of monsoons can be
divided into a number of steps:-
Step 1:
Attraction
The high
temperature conditions of the summer season leads to the development of an
extreme low pressure area extending from Pakistan to Bengal based mainly in
North India. Such a low pressure attracts the moisture bearing wind of the
South East Trades. During this time of the year there is and apparent movement
of the sun towards the tropic of Cancer due to the Summer Solstice and hence
the Inter tropical Convergence Zone shifts north. Thus the South East Trades
cross the Equator and deflect towards North East due to Coriolis force. Hence
they enter India from the South West Direction and hence are called the South
East monsoons Winds or The Advancing Monsoons.
Step 2: Onset
India receives
the bulk of its rainfall during the South West monsoon Season. Th normal Date
of the onset in Andaman and Nicobar is 20TH May, while in Kerala it
is 1ST June every year. The South West Monsoon winds are Rain
Bearing strong winds and blow at an average speed of 30 km per hour. They
usually cause Thunder and Lightning and Heavy Downpour. This sudden onset of
monsoons is called the burst of
monsoons.
Step 3:
Division
The progress of
the monsoon winds beyond Kerala is in the form of two branches i.e., The
Arabian Sea Branch and the Bay of Bengal branch.
Arabian Sea
Branch
These winds are
obstructed by the wall like mountains of the Western Ghats. The windward side
of the Sahyadris receive very heavy rainfall. The leeward side gets lesser
rainfall. Mumbai on the west coast cords about 200cm of rainfall, while Pune on
the leeward side receives only 50cm rainfall during this season. A part of the
Arabian Sea branch reaches Narmada and Tapi basin and moves further to central
India. Absence of orographic obstacles causes less rain near the coast. Another
part strikes the Saurashtra peninsula and Kachchh. Then it moves in a north
easterly direction parallel to the Arravali range in the western India. The
Arravali Range lies parallel to the direction of the monsoon winds. Thus in the
absence of an obstruction, the monsoon winds move further north without causing
much rainfall in the west. In Punjab, Haryana and other north western parts,
the Arabian Sea branch of the South west monsoons joins the Bay of Bengal
Branch. These two branches cause rains in the western Himalaya region.
The Bay Of Bengal
Branch
This branch is
directed towards the coast of Myanmar and parts of southeast Bangladesh. The
Arakan Hills in the coast of Myanmar deflect these winds towards India and
Bangladesh. Thus, the monsoon winds enter West Bengal and Bangladesh from the
South east direction instead of South west. After reaching the Himalayan
foothills it is again deflected towards the west and causes heavy rains in
Northern Plains of India. A part of the Bay of Bengal branch moves up the
Brahmaputra Valley and cause widespread rains in north-eastern India.
Mawsynram, about 16km west of Cherrapunji and located on the crest of the Khasi
hills, receives the average rainfall in the world. The rainfall in the Northern
Plains decrease from east to west. The eastern coastal Plain, particularly
Tamil Nadu, remains dry during this season. The Tamil Nadu coast lies in the
rain shadow area of the Arabian Sea Branch and lies parallel to the Bay of
Bengal branch.
The Retreat of
Monsoons
This is the
period of transition between the Final Withdrawal of the South West monsoon and
the regular setting in of the North East monsoon. With the apparent movement of
the sun southward, the low pressure area over India collapses and the monsoons
start withdrawing from India.
There is clear
sky, low humidity and the weather becomes fresh and pleasant in the north while
almost stagnant in the Deccan. This oppressive weather is called October Heat. By the beginning of
November there is fine weather conditions.
The Retreat of
Monsoons is a process much slower than its arrival. The advance of the South
West monsoons is northward while its retreat is towards the south. The retreat
of the monsoons does not imply aright about turn but a gradual change of
comparative pressure positions, thus gradually weakening and reducing the area
of coverage and influence. Kanniyakumari is incidentally the first places to
see the monsoons appear and disappear. Strong winds, torrential rainfall and
high waves along the coast are associated with the passage of tropical
cyclones. The Tamil Nadu coast and Andhra Pradesh experience heavy rainfall,
high waves and strong winds from these cyclones which are often very
destructive to life and property. They usually occur during October and
November.
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